Graded contained bulk transmission system

ABSTRACT

A graded contained bulk transmission system to transmit thousands of megawatts of electric power over distances of hundreds of miles at voltages from the order of 500,000 volts to as high as 1,500,000 volts and above. The concept is discussed in detail with respect to each phase of a polyphase system, the power in each phase being transmitted by a plurality of concentric, circular conducting cylinders the inner of which carries power at the highest voltage in the axial direction, the intermediate cylinder (or cylinders) carries power at a predetermined intermediate voltage (or voltages), and the outer cylinder is at ground potential and carries the neutral or return current for the inner conductors.

United States Patent [1 1 Wilson 1 1 GRADED CONTAINED BULK TRANSMISSION SYSTEM [75] Inventor: Gerald L. Wilson, Wayland, Mass.

[73] Assignee: Massachusetts Institute of Technology, Cambridge, Mass.

[22] Filed: Aug. 17, 1973 {21] Appl. No.: 389,075

[4 1 Mar. 4, 1975 Great Britain 174/105 R Great Britain 173/143 [57] ABSTRACT A graded contained bulk transmission system to transmit thousands of megawatts of electricpower over distances of hundreds of miles at voltages from the order of 500,000 volts to as high as 1,500,000 volts and above. The concept is discussed in detail with respect to each phase of a polyphase system, the power in each phase being transmitted by a plurality of concentric, circular conducting cylinders the inner of which carries power at the highest voltage in the axial direction, the intermediate cylinder (or cylinders) carries power at a predetermined intermediate voltage (or voltages), and the outer cylinder is at ground potential and carries the neutral or return current for the inner conductors.

7 Claims, 3 Drawing Figures PATENTED 4 I975 11 med IOI GRADED CONTAINED BULK TRANSMISSION SYSTEM The present invention relates to electric transmission systems adapted to carry power at extra-high and ultrahigh transmission voltages and, in particular, to systems wherein the high voltage elements of the system are contained or enclosed.

For a number of reasons that need not be elaborated upon here, the electric power industry has evolved into a situation wherein larger and larger blocks of power are being transmitted over larger and larger distances. The industry is faced today with a situation in which thousands of megawatts of power must be transmitted over hundreds of miles. This has prompted the thought of increasing from the 350,000 500,000 volts now used to extra-high voltages above 500,000 volts and even to ultra-high voltagesof the order of 1,500,000 volts and above. The transmission of these high voltages present multiple problems in overhead transmission, not the least of which is the allowable relatively lower power densities and, thus, poor utilization of the space occupied.

Accordingly, a principal object of the present invention is to provide an alternate to overhead lines for bulk transmission of electric power, said alternate being in the form of contained bulk transmission.

Another object is to provide a contained-bulk transmission system in which physical space occupied is greatly reduced from both overhead transmission lines and previously proposed contained bulk systems.

Another object is to provide transmission systems of the above character with virtually no radio or television interference.

Overhead lines have further difficulty in polluted atmospheres and the like of contaminated insulation flash-over; a still further object, therefore, is to remove such contamination as a problem to transmission.

Overhead lines above 500,000 volts have a further problem in that voltages induced on bodies (e.g. people and objects) can produce currents approaching lethal levels; still another object is to eliminate this latter hazard.

These and still further objects are noted hereinafter.

The objects of the invention are attained in a high voltage transmission system that comprises a plurality of concentric cylinders radially spaced predetermined distances apart, the spacing being a function of the voltage. The central cylinder of the plurality of cylinders is connected to a source of high transmission level voltage, to carry current and thus transmit power in the axial direction. A further cylinder also carries electric current in the axial direction. In the system discussed in greatest detail herein there is an outer cylinder spaced outward from the further (or intermediate) cylinder, and the outer cylinder is connected to ground and carries neutral or ground current. A polyphase system can be made by combining the nested unit consisting of the concentric cylinders with other like units. But, it should be noted, a three-phase system can be constructed, for example, with one outer cylinder and three pairs of inner cylinders appropriately spaced.

The invention is hereinafter described with reference to the accompanying drawing in which:

FIG. 1 is a section end view, looking in the axial direction, of a high voltage transmission line comprising a solid center cylinder, an intermediate cylinder radially spaced outward a predetermined distance from the center cylinder and an outer cylinder spaced a predetermined distance from the intermediate cylinder;

FIG. 2 is a section view taken upon the line 22 in FIG. 1, looking in the direction of the arrows, and shows, among other things, sections of annular insulators that separate and support the nested cylinders; and

FIG. 3 is a schematic circuit diagram showing a transmission line made up of nested cylinders, a transformer at each end of the line, and reactors at each end connected to both the center cylinder and the intermediate cylinder to supply, in part, reactive power requirements of the line.

Turning now to the figures, a transmission system is shown at 101 for conducting electric power at extrahigh to ultra-high voltages from the transformer labeled 20 in FIG. 3 to the transformer labeled 2]. The distance between the transformers can be hundreds of miles, the power carried by the system can be thousands of megawatts, and the transmission voltages can be from the order of 500,000 to as high as 1,500,000 volts, and above. Power is transmitted between the two transformers of the system axially along a transmission line 22 comprising nested, coaxial, circular conducting cylinders, as now discussed generally and as later discussed in." greater detail.

The transmission line 22 comprises a center cylinder 1 connected to the high voltage taps shown at 7 and 7 of the transformers 20 and 21, respectively, thereby to supply current thereto, the highest transmission level voltage being connected to the cylinder 1, of aluminum or the like, which carries electric current and thus transmits electric power in the axial direction. An intermediate cylinder 2, disposed radially outward from the central cylinder 1, is connected to the transformers 20 and 21 at the taps labeled 8 and 8', respectively, the taps 8 and 8 being at a voltage level that is a predetermined voltage whose level is less than that at the taps 7 and 7. An outer conducting cylinder 3 is disposed radially outward from the intermediate cylinder 2. The radii r r and r respectively of the center cylinder 1, the intermediate cylinder 2 and the outer cylinder 3 are determined, as later discussed, by the voltage applied to each (the cylinder 3 is at ground; ground designates actual earthing in most situations as well as a common system connection), and the radii r r and r are chosen to provide an optimum electric field in the regions labeled I and II between the cylinders 1-2 and 2-3, respectively.

Although the transmission line 22 shown in FIGS. 1-3 has one intermediate cylinder and operating potential for simplicity, there may be several intermediate cylinders with corresponding intermediate but unequal electric potentials. The principle here is to reduce the overall size of the transmission line or device 22 by carefully controlling the electric fields between these where E is the critical or maximum electric field, 8 is proportional to the pressure of the dielectric gas belm/r (Region I) 110) 2L' 2/ egion II) where E and E are the fields at the center of the inner and intermediate conductors and r is the radial distance to the point between the cylinders where the 1 electric field E(r) is desired to be calculated.

The maximum voltage between each pair of cylinders" that will exist without electrical breakdown of the di-' electric between FhEPXlPEM9BElPX int rat n 2 B 0(A r l 2 1 V V V i crit opt crit opt A 2 2(5r (A r I equations (2) while setting theelectric field at the inner cylinder equal to the critical value given by equation (1). Thus the circuit or sparkover voltages for the regions l and II are given by crit.

cri

ll the values given belowin the expressions() and crit optimum B crlt optimum T he design procedure then is as follows. First a choice is made of the desired or required breakdown voltage (V between the innermost cylinder 1 and outermost :cylinder 3. This wouldcorrespond to the 50% flashover voltage foran overhead system. Then Equation (8) can be combined with equation (5) to yield a value for-the radius r for the value of V, chosen. The intermediate radius r is then found from equation (5); the outer radius r;, is found from equation (7); and the optimum critical voltage for the intermediate conductor is found from equation (6). Using the results from switching surge studies as is found for conventional lines performed on a model ofthe system shown in FIG. 3, the ratio of the 50% flashover intermediate voltage to the operating voltage can be determined and the design is complete.

As an example of the improvement of this device over existing systems, consider a typical 765 kV transmission system. This consists of a right-of-way 200 feet wide with towers feet high. A contained bulk graded transmission system of the present invention designed with an 1800 kV line-to-neutral peak 50% flashover level would require three pipes, each with 7.7 feet outer diameter, 2.59 feet intermediate diameter operating at 560 kV and 0.9 feet inner diameter operating at 765 kV. This gives a cross sectional area reduction over that of an overhead line of almost four orders of magnitude. If the system did not employ grading, the required outer diameter would be 10 feet. If the transmission line 22 is designed to operate at two atmospheres of compressed air, the graded system just described would have an outer diameter of 3.9 feet. For operation in air, the heat transfer between the cylinders would be accomplished by natural convection to the outer cylinder. The outer surface of .the outer cylinder would have fins placed on the upper surface to enhance n u l. nvection. h at ans r. E9 ..l. .L.l[ QPh A few pertinent bits of information are contained in this and the next two paragraphs. In an air system the values of A and B in the foregoing expressions are 31 KV and 9.55 KV, respectively, and 8 is l for air at atmo'spheric pressure and standard temperature. The term cylinder 1 is a solid cylinder but need not be; the cylinders 2 and 3 are shells, the radii r and r being taken from the axis to the center of the shell whose thickness is small compared to the cylinder radius.

The major dielectric between the cylinders is a gas, either air or some other gaseous dielectricat atmospheric pressure or at elevated pressures. The cylinders are supported by the insulating dielectric designated 4, 4 5,5 at appropriate locations along the axial direction. These may consist of porcelain or other appropriate dielectric material such as a phenolic. The transmission structure 22 of cylinders is energized at its sending and receiving ends by the transformers 20 and 21 whose primary sides are, in an operating system, interconnected to an electric power network. The secondary sides which feed the transmission structure 22 operate at multiple electric potentials supplied by taps, as mentioned, the highest potential taps 7 and 7 feeding the center cylinder 1 of the transmission structure and the intermediate potential taps 8 and 8' feeding the intermediate cylinder 2 lying between the center and outer conductors. The reactive power requirements of the structure are supplied, in part, by shuntcompensating reactors 9, l0, 9' and 10" connected to both the center and intermediate cylinders of the structure, as shown.

A polyphase transmission system, can be made by combining a plurality of the systems 101 in an appropriate configuration, as above noted. it should be further noted that a three-phase system, for example, can be made by placing three pairs of concentric cylinders, like the cylinders 1 and 2, in an appropriate configuration, properly insulated and held, with one outer cylinder, like the cylinder disposed about and enclosing the three pairs and carrying any neutral or return current that the system may require. The insulators 4 etc. need not be annular, but may be of some other design. The system is particularly adapted for systems above 500,000 volts, but conceptually it can be used below such voltage.

Further modifications of the invention herein disclosed will occur to persons skilled in the art and all such modifications are deemed to be within the spirit and scope of the invention as defined by the appended claims.

What is claimed is:

l. A transmission system adapted to transmit power at extra-high to ultra-high voltages, that comprises: a plurality of concentric, circular, conducting cylinders, the center cylinder of the plurality being adapted to connect to a source of high transmission level voltage, to carry current and thus transmit electric power in the axial direction, and an intermediate cylinder disposed radially outward from the center cylinder and adapted to connect to a source of voltage whose voltage level is less than that connected to the center conductor, a gaseous dielectric medium under pressure occupying the region between the two cylinders, the radius r, of the center cylinder and the radius r of said further cylinder being found from the following two expressions:

crit optimum and having a radius r;, found from the following express1on:

the optimum allowable voltage that can be applied between said intermediate cylinder and the outer cylinder being found from the following expression:

2 B 6(A r 2 l72 2 crit: optimum A 3. A transmission system as claimed in claim 2 that further includes a source of voltage, a hightransmission level voltage from said source being connected to the center cylinder and a lower voltage from said source being connected to the intermediate cylin- .der.

4. A transmission system as claimed in claim 3 in iwhich there are several intermediate coaxial cylinders between the center cylinder and the outer cylinder with corresponding intermediate but unequal electric potentials, the intermediate cylinder closest to the center cylinder being at the next highest voltage to that of the center cylinder and the further intermediate cylinder or cylinder being at a lower predetermined voltage level or levels.

5. A transmission system as claimed in claim 2 in which the center cylinder and the intermediate cylinder are energized at both the sending end and the receiving end, a tapped transformer being connected at each end to effect energization, the high voltage tap of the transformer being connected to the center cylinder and a lower voltage tap of the transformer being connected to the intermediate cylinder.

6. A transmission System as claimed in claim which cylinders between the center cylinder and the out er cylinder, the voltage level connected to the intermediate cylmders closest to the center cylinder being less than the voltage connected to the center cylinder and greater than the voltage connected to the next radially outwardly disposed cylinder. 

1. A transmission system adapted to transmit power at extra-high to ultra-high voltages, that comprises: a plurality of concentric, circular, conducting cylinders, the center cylinder of the plurality being adapted to connect to a source of high transmission level voltage, to carry current and thus transmit electric power in the axial direction, and an intermediate cylinder disposed radially outward from the center cylinder and adapted to connect to a source of voltage whose voltage level is less than that connected to the center conductor, a gaseous dielectric medium under pressure occupying the region between the two cylinders, the raDius r1 of the center cylinder and the radius r2 of said further cylinder being found from the following two expressions:
 2. A transmission system as claimed in claim 1 that includes an outer cylinder coaxial with the other two and having a radius r3 found from the following expression:
 3. A transmission system as claimed in claim 2 that further includes a source of voltage, a high-transmission level voltage from said source being connected to the center cylinder and a lower voltage from said source being connected to the intermediate cylinder.
 4. A transmission system as claimed in claim 3 in which there are several intermediate coaxial cylinders between the center cylinder and the outer cylinder with corresponding intermediate but unequal electric potentials, the intermediate cylinder closest to the center cylinder being at the next highest voltage to that of the center cylinder and the further intermediate cylinder or cylinder being at a lower predetermined voltage level or levels.
 5. A transmission system as claimed in claim 2 in which the center cylinder and the intermediate cylinder are energized at both the sending end and the receiving end, a tapped transformer being connected at each end to effect energization, the high voltage tap of the transformer being connected to the center cylinder and a lower voltage tap of the transformer being connected to the intermediate cylinder.
 6. A transmission system as claimed in claim 5 which includes shunt compensating reactors connected to both the center cylinder and the intermediate cylinder to supply reactive power requirements of the system.
 7. A transmission system as claimed in claim 3 in which there is a plurality of intermediate conducting cylinders between the center cylinder and the outer cylinder, the voltage level connected to the intermediate cylinders closest to the center cylinder being less than the voltage connected to the center cylinder and greater than the voltage connected to the next radially outwardly disposed cylinder. 